[AMDGPU] Add support for a16 modifiear for gfx9

Summary:
Adding support for a16 for gfx9. A16 bit replaces r128 bit for gfx9.

Change-Id: Ie8b881e4e6d2f023fb5e0150420893513e5f4841

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D50575

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340831 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed